cmd/gc: fix a spurious -u compile error

Fixes #4082.

R=dsymonds
CC=golang-dev
https://golang.org/cl/6545055

src/cmd/gc/go.h

@@ -919,6 +919,7 @@ EXTERN	int32	thunk;
 EXTERN	int	funcdepth;
 EXTERN	int	typecheckok;
 EXTERN	int	compiling_runtime;
+EXTERN	int	compiling_wrappers;
 
 /*
  *	y.tab.c

src/cmd/gc/reflect.c

@@ -210,22 +210,26 @@ methods(Type *t)
 				// but we can generate more efficient code
 				// using genembedtramp if all that is necessary
 				// is a pointer adjustment and a JMP.
+				compiling_wrappers = 1;
 				if(isptr[it->etype] && isptr[this->etype]
 				&& f->embedded && !isifacemethod(f->type))
 					genembedtramp(it, f, a->isym, 1);
 				else
 					genwrapper(it, f, a->isym, 1);
+				compiling_wrappers = 0;
 			}
 		}
 
 		if(!(a->tsym->flags & SymSiggen)) {
 			a->tsym->flags |= SymSiggen;
 			if(!eqtype(this, t)) {
+				compiling_wrappers = 1;
 				if(isptr[t->etype] && isptr[this->etype]
 				&& f->embedded && !isifacemethod(f->type))
 					genembedtramp(t, f, a->tsym, 0);
 				else
 					genwrapper(t, f, a->tsym, 0);
+				compiling_wrappers = 0;
 			}
 		}
 	}

src/cmd/gc/typecheck.c

@@ -1536,9 +1536,7 @@ ret:
 		}
 	}
 
-	// TODO(rsc): should not need to check importpkg,
-	// but reflect mentions unsafe.Pointer.
-	if(safemode && !incannedimport && !importpkg && t && t->etype == TUNSAFEPTR)
+	if(safemode && !incannedimport && !importpkg && !compiling_wrappers && t && t->etype == TUNSAFEPTR)
 		yyerror("cannot use unsafe.Pointer");
 
 	evconst(n);